The present invention relates to an optical head for applying a focused light beam to an information memory medium, thereby recording or retrieving information optically.
Apparatuses and information memory media have recently been developed which are used to record or retrieve information optically. The memory media include compact disks for DAD, video disks, optical cards, or other retrieval-only memory media. The media further include those information memory media for document files, computer output memories, etc., which are used for recording and retrieval only, or which permit recording, retrieval, and erasure of information. These optical information recording/retrieving apparatuses incorporate an optical head, which applies a light beam to an information memory medium, thereby optically recording on or retrieving information from the medium. In this optical head, a laser beam emitted from a semiconductor laser (light source) is focused on the memory medium by an objective lens, and the reflected beam from the medium passes again through the objective lens, and is then detected by a photodetector. A detection signal is processed and converted into an information signal. In the optical head of this type, the light transmitted through the objective lens must be focused exactly on a record film of the information memory medium, and a tracking guide of the medium must be traced by the focused laser beam. To attain this, the optical head is provided with a focusing servo loop to detect and maintain a focusing state of the objective lens, and a tracking servo loop for detecting the tracking guide, and continually directing the objective lens toward a desired track.
The focusing state of the objective lens may be detected by the knife-edge method, as stated in U.S. Pat. Nos. 4,517,666; 4,546,460; 4,585,933 and 4,521,680 (by Ando). The tracking guide may be detected by a method such that a diffraction component diffracted by the tracking guide is detected, as stated also in the aforesaid U.S. patents. According to optical heads disclosed in these patents, a light shielding plate or a so-called knife edge is located on the optical path. The knife edge must be positioned with high accuracy, and use of it increases the number of components that must be assembled. Thus, the assembly and adjustment require a lot of time, and the resulting optical system is inevitably large-sized.
To cope with these problems, improved optical systems have been proposed in U.S. Pat. No. 4,654,519, and U.S. Pat. Appl. Ser. No. 862,829, filed on May 13, 1986 (both by Ando). In the systems disclosed in these applications, a prism is used as a beam splitter, which has a pair of beam-emerging surfaces, relatively inclined and bonded to each other on a boundary line. In these systems, one of the beam-emerging surfaces serves as a knife edge. A light beam emerging from the other emerging surface is directed to a photodetector for focusing detection, while a light beam emerging from the first emerging surface is directed to a photodetector for tracking-guide detection, located within a plane different from that of the focusing-detection photodetector. Signals from the photodetectors are processed into focusing and tracking signals. Not provided with any light shielding plate, the proposed systems are advantageous in that their components are relatively few, and can be assembled and adjusted with ease. Since the photodetectors for focusing detection and tracking-guide detection are arranged separately, however, the prior art optical systems are still complicated in structure, bulky, and heavy in weight.